Workpieces can become contaminated with titanium dioxide in several ways. Because titanium dioxide is a widely used white dye, many processes that involve dyeing objects or fabrics white will result in contaminated machinery, containers, and filters. Titanium dioxide scale frequently forms on titanium metal during working to produce objects made of the metal. In particular, filters used in the manufacturing process of polyester fabric that is dyed white become contaminated with titanium dioxide.
Titanium dioxide is used in the manufacture of polyester fiber primarily as a dye, and secondarily to prevent certain unwanted properties inherent in raw polyester fabric. During the manufacturing process, the liquid polyester is filtered, contaminating the filter media with organic compounds and titanium dioxide. The filter media is typically a fine stainless steel wire mesh nickel-brazed onto a base. During cleaning, the filter is subjected to a fluidized bath containing hot aluminum oxide particles to remove organic compounds. Titanium dioxide, aluminum oxide and residual organic compounds contaminate these filters after the initial cleaning, and are removed by the present invention.
The present invention utilizes calcium ion in conjunction with an alkaline solution which inhibits the formation of the insoluble, gelatinous sodium hydrogen (me) titanate (NaHTiO.sub.3). The present invention provides an effective method for removal of titanium dioxide from contaminated workpieces under conditions that are exceedingly mild compared to conditions used in conventional techniques. Delicate workpieces can be cleaned using this method without damage.
The conventional methods for removing titanium dioxide and residual organic compounds require immersing the contaminated workpiece in hot, concentrated alkaline solution. Sodium hydroxide reacts with titanium dioxide to form sodium hydrogen titanate, a gelatinous substance that is virtually impossible to remove by mechanical techniques. It is the removal of this intractable substance that requires the rigorous conditions of conventional techniques. Examples of conventional techniques include those recited in U.S. Pat. Nos. 3,690,949 and 2,790,738. A method is disclosed in U.S. Pat. No. 3,690,949 involving immersing a contaminated workpiece in a hot (200.degree. to 300.degree. C.) solution of 50% sodium hydroxide, with small amounts of sodium nitrate, sodium nitrite, and sodium gluconate added. In U.S. Pat. No. 2,790,738 a method is disclosed that requires immersing the workpiece in molten alkali metal hydroxide, heated to approximately 700.degree. F. Hot, concentrated alkali metal hydroxide will attack not only the stainless steel filtering media used in polymer fiber formation, but also the brazed connection of the filtering media to the base.
Recognizing the need for a more effective method to clean workpieces contaminated with titanium dioxide, and in particular a method that won't damage delicate workpieces due to conditions required to remove titanium dioxide, the present invention provides a method of removal of titanium dioxide utilizing mild conditions.
Titianium dioxide is removed from workpieces by immersing the workpieces in an alkali solution containing calcium ion. Calcium ion, in the form of a soluble calcium salt, is added to a 30% by weight aqueous sodium hydroxide solution. Typically, it is found that the calcium available for solution must be present in at least 2-fold molar excess compared to the titanium present in the titanium dioxide to be removed. The temperature of operation is optimally 30.degree. C. Approximately 0.5 ml surfactant per liter of solution may be added. After 30 minutes of soaking, the workpiece is removed and rinsed, e.g., in water.